The concept of utilizing electromagnetic waves to study porous media, including geological materials, has existed for some time and, more particularly, dates back to Maxwell in (1881) "A Treatise on Electricity and Magnetism", 2nd Ed. Clarendon Press, Oxford, England, 398.
The use of electromagnetic waves in geo-applications is attractive for two reasons. Firstly, all factors that effect an electrical response of a soil due to excitation by an alternating (AC) voltage derive from electrical, chemical, and mechanical characteristics of the soil itself. Secondly, the application of AC voltage to soil and geological media does not alter the properties thereof. It is, therefore, possible to employ an electrical response that constitutes a reflection of the properties of the soil in its in situ state. As has been noted by Schwartz (1962) "A Theory of the Low-frequency Dielectric Dispersion of Colloidal Particles in Electrolyte Solution", J. Phys. Chem. 66, 2636-2642; and Mitchell and Arulanandan (1968) "Electrical Dispersion in Relation to Soil Structure", ASCE, J. Soil Mech. and Found. Div., 94(2), 447-470, due to the mechanical and chemical interactions within soils at the micro level, the macroscopic electrical response in terms of conductance and capacitance of the soil in bulk are frequency dependent. Since the same factors control macroscopic mechanical soil behavior, an electrical response is required to provide a response spectra of soil-signature information at a microscopic as well as at the macroscopic level. That is, such a dispersion of measured frequency-dependent electrical parameters reflect the microscopic physical and chemical soil interactions as well as macroscopic bulk soil behavior and, thereby, can provide a spectrum of soil-signature information.
The above is meaningful as a point of departure of research into frequency domain analysis of electrical dispersion characteristics of reflected microwaves in the context of numerous geological and geo-environmental issues of concern to civil, chemical, environmental, petroleum, and other engineers. More particularly, geo-technical and geo-environmental problems often appear in the context of so-called two-phase soil, that is, soil consisting of a solid particle portion and a fluid portion. It is to the analysis of such two-phase soil that the instant method of data interpretation is directed.
A two-phase soil model includes various soil types having concentrations of pore fluids and solids including therein water, air, solutes, oil, and various pollutants or contaminants, and having solid particles of various sizes, shapes, arrangements, and inter-particle bondings. Accordingly, the present area is to be understood as a method of electrical dispersion data interpretation by which parameters of soil particle shape, particle size, particle orientation, soil porosity, water content, non-water content, and contaminant concentration, liquid phase conductivity and dielectric constant, solid phase conductivity and dielectric constant, and liquid-solid phase interface may all be derived from such a frequency domain analysis of electrical dispersion of porous media.
The instant invention is, therefore, a recognition that, due to mechanical and chemical interactions of soil at the micro level, and the hetrogenity in the electrical parameters of the basic constituents of soil, the macroscopic electrical response in terms of conductance and permittivity (dielectric constant) of the bulk soil are frequency dependent. The instant invention may therefore be viewed as a means of quantifying of the effect upon measured electrical dispersion of bulk soil upon the specific parameters of particle shape, particle size, particle distribution, porosity, water content, other fluid content, type of soil, conductivity and dielectric and interface surface fluid-particle interface characteristics.
The invention is, accordingly, to be understood as a non-invasive, non-destructive in situ technology for providing reliable interpretation needed to quantitatively predict and evaluate basic factors that control soil behavior, define the presence and type of pore fluid including hydrocarbon and its contaminants, monitor contaminant transport, and monitor post clean-up ground water and soil conditions.
With respect to the applicable prior art, Schwartz (1962), cited above, investigated possible mechanisms causing anomalous dielectric dispersion in frequency ranges of under 100 KHz and concluded that basic soil-water interfacial electrical parameters are frequency-dependent and cause low frequency electrical dispersion. Mitchell and Arulanandan (1968), cited above, investigated variations in conductivity and dielectric constant over frequency ranges under 100 MHz for clays and concluded that the magnitude of the dispersion was related to soil type.
Sachs and Spiegler (1964) "Radiofrequency Measurements of Porous Conductive Plugs" Ion Exchange Resin-Solution Systems, Journal of Physical Chemistry, Vol. 68, p1214, investigated anomalous dispersion in the radio-frequency range (one to three hundred MHz) and developed an empirical equivalent three-element circuit model to match the observed dispersion, Arulanandan and Smith (1974) "Electrical Dispersion in Relation to Soil Structure", J. Geotech. Eng Div., ASCE, 99(12), 1113-1133 evaluated the applicability of this circuit model to soils and attempted to provide explanations for the circuit model of Sachs and Speigler. The empirical nature of this model provided varying degrees of success in matching observed dispersions. However, the empirical model parameters obtained, using curve fitting procedures, could not be linked to soil composition and other parameters on a reliable or fundamental basis. Further, the model of Sachs and Speigler was one dimensional and, as such, was useful only with measurements that could be taken from a single physical direction or plane.
Sen et al (1981) "A Self-similar Model for Sedimentary Rocks with Application to the Dielectric Constant of Fused Glass Beads" Geophysics, 46(5), 781-795, developed a semi-theoretical model to study dielectric response of water-saturated rocks. However, they concluded that their model could not fully simulate observed dispersions. Similar attempts were made by Kenyon (1984) "Texture effects of Megahertz Dielectric Properties 3153-3159, and Raytha and Sen (1986) "Dielectric Properties of Clay Suspension in MHz to GHz Range", J. Colloid and Interface Sc., 109(2), 301-309. They also concluded that their models could not simulate observed dispersion data of different salinities in the fluid phase.
The present inventor began his investigation of electrical characterization of soil properties in 1987 at Purdue University. The earliest publication in connection therewith of the inventor occurred in 1991. See "Level Ground Soil-Liquefaction Analysis Using In-Situ Properties: I", ASCE, J. Geotech Eng Div., 117(2), p.364-367. In 1993 (Apr. 6 to 8), the inventor presented a further development of his ideas concerning electrical response of two phase soil at the ENPC Conference, Paris, France. See "Soil Pore Fluid Characterization Using Electromagnetic Waves", p.285-292, ENPC Proceeding 1993. The subject matter of said ENPC presentation was also incorporated into lectures of the inventor of Jun. 6-9, 1993, at the University of Virginia, ASCE/ASME/SES meeting of 1993, and at the Jun. 11-12, 1993, NSF workshop on Geo-Physical Techniques For Site and Material Characterization in Atlanta, Ga. In Aug., 1993 the inventor's paper entitled "Electrical Response to Two-Phase Soil: Theory and Applications" was published by ASCE J. Geotech. Eng Div. 119 (8) p.1250-1275.
While the inventor's above set forth papers discuss the theoretical basis of the instant invention, the implementation, i.e., reduction to practice thereof is not addressed therein.
With respect to the hardware, that is, the electromagnetic probe utilized to practice the present invention, such structures appear in the prior art as are taught in U.S. Pat. No. 4,654,598 (1987) to Arulanandan, et al, entitled Dielectric Method and Apparatus for In Situ Prediction of Porosity and Surface Area; and U.S. Pat. No. 4,866,371 (1989) to De, entitled Sample Accommodator and Method for the Measurement of Dielectric Properties.